Vehicles may be fitted with evaporative emission control systems to reduce the release of fuel vapors to the atmosphere. For example, vaporized hydrocarbons (HCs) from a fuel tank may be stored in a fuel vapor canister packed with an adsorbent which adsorbs and stores the vapors. At a later time, when the engine is in operation, the evaporative emission control system allows the vapors to be purged into the engine intake manifold for use as fuel. Further, such evaporative emission control systems may be monitored during certain conditions to detect degradation of system components, such as leaks, for example.
The inventors herein have recognized that an increasing number of vehicle application are having issues with canister purge due to lower available engine vacuums, such as in stop/start and hybrid applications. Approaches are known which use a two way (open/closed) canister vent valve and an open venting system where the vent valve is in a normally open position during diurnal conditions, such as during refueling and other engine off conditions. The inventors herein have recognized that approaches which use a normally open canister vent valve, where the vent valve is positioned in a fully open position, during diurnal conditions may result in reduced vapor storage capacity and an increased an amount of vacuum needed to effectively purge the canister. Further, in some examples, such approaches may rely on fuel tank isolation valve to effectively monitor and purge the system which may increase costs.
In order to address these issues, in one example approach a method for an engine with a fuel vapor recovery system is provided. The method comprises, during a diurnal condition, providing a first amount of venting to a fuel vapor canister, and during a purge condition, providing a second amount of venting to the fuel vapor canister, where the second amount is greater than the first amount.
In this way, vapor generation in a fuel vapor recovery system may be reduced and vapor storage capacity of the canister may be more effectively utilized by restricting the canister fresh air vent during diurnal events. For example, by restricting the fresh air path at the canister vent during diurnal events and allowing the evaporative system to build positive pressure, vapor generation may be reduced and an increased amount of pressure may be available during purging and monitoring conditions without relying on costly valve systems. Also, by restricting the canister vent, vapor may be forced to travel over the canister bed at a slower rate allowing more residence time within the canister which allows the canister bed material to bond more vapor in the bed before reaching the atmosphere.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.